The advancement of technology, in any industry, requires the replacement of older equipment with newer, faster, and more efficient equipment. In the telephone industry, for example, older telephone switches that function to complete the circuit between a customer's telephone (talk circuit) and the telephone company's central office, are continually being replaced by more inexpensive, efficient, and higher capacity switches. For example, FIG. 1(a) illustrates a conventional communications switching circuit, e.g., a customer telephone, connected to the telephone company switch 15 through a Main Distributing Frame ("MDF"). The switch 15 includes a -48 volt source which is connected to the "Ring" connection through a 200 ohm impedance and passed through to the "Ring" connection of the talk circuit through the MDF connector 20. A return path from the talk circuit is provided on the "Tip" connection which is passed through the MDF connector 22 to ground through a second 200 ohm impedance. As shown in FIG. 1(a), there is a voltage of approximately -48 V between the tip and ring during customer equipment "on-hook" conditions, when there are no paths present for the office power to flow through the customer loop. During "off-hook" conditions there is a path provided by the customer loop for a current I.sub.LOOP to flow (typically through a telephone, modem, or other telecommunications equipment.) Typical voltages between customer tip and ring during "off-hook" conditions vary from about -7.0 Volts to -48 Volts.
Currently, the older switching units such as, e.g., the 1ESS/1AESS/2BESS (products of Lucent Technologies, assignee of the instant invention) for switching several thousand connections to the MDF of a telephone central office, are being replaced with newer, more functional, high capacity switch 30, e.g., model 5ESS (product of Lucent Technologies) having 10,000 or more connections. As shown in FIG. 1(b), the installation of the new switch 30, into the existing switching circtuit structure of FIG. 1(a) requires the splicing of a connecting lead, e.g., leads 32a and 32b, from the 5ESS switch to the corresponding leads, e.g., 23a and 23b of the older switch 15, e.g., 1AESS switch, utilizing a splicing connector block 25, e.g., a 711 connector. Before removing the connections of the older switch 15, verification must be made to ensure that all of the spliced connections, and hence, all of the possible circuit connections between the MDF and the new switch 30, are complete. This essentially entails an in-service wiring verification technique, and test sets have been built to accomplish this task.
One prior art in-circuit verification test system, the "Minitrace" Single Lead Continuity Tester ITE-6157, manufactured by the C.E.Cox Company, provides in-service wire verification/continuity tests on a single lead at a time, and requires two installers to simultaneously probe a wire from opposite ends while listening for a verification tone to be heard. If no tone was heard, an indication is provided that an open wire exists or that the connected wires were on different leads. No provision was made for testing the possibility of adjacent shorts and if either installer makes an error in placement, continuity will not be detected. This procedure was slow, less accurate (shorts could not readily be found, and opens and reversals were difficult to interpret), and additionally was prone to installer operation error because of fatigue from hours of repetitious work.
U.S. Pat. No. 4,130,794 to Cox describes another early in-circuit tester that provides a voltage source enabling continuity detection in otherwise unpowered circuits. These prior art testers were, at best, semi-automated, and a fully automated in-circuit testing solution is highly desirable.